The mutD gene is a strong Escherichia coli mutator. mutD cultures typically exhibit mutation rates of 10 to the -5 to 10 to the -4 per base-pair replication. The mutator phenotype depends on growth conditions. The mutation rate is low in cultures propagated in minimal-salts medium, high in minimal supplemented with thymidine. The mutD region of the chromosome codes for a 28,000MW protein. This proposal presents various methods for analyzing the structure and function of the mutD region. It focuses on genetic methods for isolating new mutations in mutD and suggests ways in which the mutD promoter can be fused to Beta-galactosidase, and the lacZ promoter to the 28kD protein. Experiments are proposed to use the fusion plasmids to understand mutD regulation and to purify the mutD protein.